In a residential, commercial or industrial electrical system, electrical power is typically delivered from a utility source to a panelboard. Electrical power is then provided via the panelboard to one or more branch circuits, each of which supplies power to one or more loads. Various types of protective devices can be installed on the panelboard to reduce the likelihood of damage from excessive heat or fire, for example, as well as reduce risk of injury from electric shock.
Circuit breakers are generally provided for each branch circuit on a panelboard for overload protection and short circuit protection. Overload occurs when too many loads simultaneously draw power from the same branch circuit, or when a single load draws more power than the branch circuit is designed to carry. A circuit breaker can be provided with a thermal element, which becomes heated by an increase in current from an overload condition. The thermal element causes the circuit breaker to trip and interrupt power when heated beyond a predetermined threshold. With regard to short circuit conditions, a circuit breaker can be provided with an electromagnetic device for sensing high current flow and for tripping the circuit breaker and thereby interrupting power when current exceeds a predetermined threshold.
Many circuit breakers are designed to protect against ground faults, that is, when current flows from a hot conductor to ground through a person or an object. Such current flow can be dangerous to a human being because it can cause heart fibrillation. Circuit breakers which operate as ground fault circuit interrupters (GFCIs) are therefore designed with a fast response time to trip and interrupt power very quickly upon detection of a ground fault condition.
Arcing faults are commonly defined as current through ionized gas between two ends of a broken conductor, between two conductors supplying a load, or between a conductor and ground. A number of conditions can cause an arcing fault such as corroded, worn or aged wiring or insulation, a loose connection, wiring damages by a nail or staple in the insulation, and electrical stress from repeated overload conditions or lightning, among other conditions. A problem exists in many electrical distribution systems because arcing faults do not cause the corresponding circuit breaker to trip. Arcing fault current levels are sometimes reduced by branch circuit or load impedance to a level below the trip curve settings of the circuit breaker for that branch circuit. Further, an arcing fault which does not contact a ground conductor or person may not trip a GFCI. Accordingly, a number of existing electrical distribution systems employ a circuit breaker and a separate arcing fault detection device for each branch circuit on a panelboard, resulting in unnecessary redundancy in panelboard components and their functions. For example, FIG. 1 depicts an existing electrical distribution system which provides an arc detector 10a, 10b, . . . 10n and a circuit breaker or line interrupter 22a, 22b, . . . 22n for respective ones of a plurality of branch circuits 1a, 1b, . . . 1n.
A need exists for a fault protection device for an electrical distribution system which detects ground faults and arcing faults with improved accuracy. A need also exists for a fault detection device for an electrical distribution system which reduces redundancy of protective devices on a panelboard.